In a radio cellular communication system, a random access procedure is used when a terminal (also called as a user equipment UE) in an idle state (RRC_IDLE) initially accesses a network or a terminal in a connected state (RRC_CONNECTED) synchronizes with a network and acquires resource allocation for subsequent data communication.
In a 3rd generation mobile communication long term evolution (LTE) system, the following five events can trigger a random access procedure: (1) initial access in an idle state; (2) initial access after radio link failure (RLF); (3) handover (HO); (4) downlink data arrival in a connected state; and (5) uplink data arrival in a connected state. Moreover, there are two different types of the random access procedure: a contention-based procedure (applicable for all the five events) and a non-contention-based procedure (only applicable for event (3) and event (4) as above). After the success of the random access procedure, normal downlink or uplink transmission may begin.
A contention-based random access procedure is shown in FIG. 1, which includes four steps.
Step 1: a terminal transmits a random access preamble through an uplink random access channel (RACH).
Step 2: a medium access control (MAC) layer of a base station generates a random access response message, which is sent to the terminal over a downlink shared channel (DL-SCH).
This message at least includes a random access preamble identifier (RAPID), time alignment (TA) information, initial uplink grant (UL grant) and a temporary cell-radio network temporary identifier (Temporary C-RNTI), and this message is indicated through a random access-radio network temporary identifier (RA-RNTI) on a physical downlink control channel (PDCCH).
Step 3: the terminal sends a first scheduled transmission message on an uplink-shared channel (UL-SCH).
This message at least includes a cell-radio network temporary identifier (C-RNTI), an MAC element, or a common control channel service data unit (CCCH SDU), and the transmission of this message supports a hybrid automatic retransmission request (HARQ).
Step 4: the base station sends a contention resolution message on the DL-SCH.
This message is indicated through a C-RNTI or a temporary C-RNTI on the PDCCH, and the transmission of this message supports an HARQ.
A non-contention-based random access procedure is shown in FIG. 2, which includes the following three steps.
Step 0: a base station allocates a random access preamble to a terminal through a downlink dedicated signaling.
This signaling is generated by a target base station in the condition of handover and transmitted to the terminal by a source base station through a handover command (HO command); or it is transmitted to the terminal through a PDCCH if downlink data have arrived.
Step 1: the terminal transmits the allocated non-contention-based random access preamble through an uplink random access channel (RACH).
Step 2: the base station transmits a random access response message on a downlink-shared channel (DL-SCH).
This message at least includes time alignment information, a random access preamble identifier, and also includes initial uplink grant information in the condition of handover; and this message is indicated through a random access radio network temporary identifier (RA-RNTI) on the PDCCH.
The following time relations exist in the relational steps of the above random access procedure.
(1) Time for Transmitting the Random Access Preamble
As regards the time for transmitting the random access preamble in step 1, it is determined by the terminal according to the time for occurrence of the aforesaid five triggering events and/or a backoff value (overload indicator) stored by the terminal itself. The backoff value is initialized to 0. The base station may send a backoff value to the terminal via the random access response message in step 2 to determine the time for retransmission of the random access preamble in case of failure of the subsequent random access procedure. That is to say, the time for retransmission of the random access preamble, i.e. the time for transmitting the random access preamble next time depends on initial time of triggering the random access procedure and one backoff value. The typical configuration of the backoff value includes (0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 500, 1000), taking the millisecond (ms) as a unit. As regards the non-contention-based random access procedure triggered because of downlink data arrival, the time interval is 4 ms between the moment in step 0 that the terminal receives a subframe containing a PDCCH and the moment in step 1 that the terminal initially transmits the random access preamble.
(2) Random Access Preamble-Random Access Response
For a frequency divided duplex (FDD) mode, at 2 ms (the typical backoff length is 4 ms, and 2 ms is selected for reliability) after transmitting the random access preamble in step 1, the terminal monitors the RA-RNTI on the PDCCH in the time range of a transmission time interval window (RA_WINDOW_BEGIN-RA_WINDOW_END) (also called a random access response window) to receive the random access response message in step 2. The typical configuration of the length of the random access response window is from 2 ms to 10 ms.
For a time divided duplex (TDD) mode, the delay length after which the random access preamble is transmitted to the first subframe of the random access response relates to specific downlink/uplink subframe allocation. If the terminal successfully monitors the corresponding RA-RNTI on the PDCCH in the random access response window, and the random access preamble identifier contained in the random access response message corresponds to the random access preamble transmitted by the terminal, then the random access response is regarded as successfully received. The terminal may stop monitoring the random access response message after the random access response message is successfully received. For the non-contention-based random access procedure, it also means that the random access procedure is successful. If the terminal fails to receive the random access response message in the random access response window, or any random access preamble identifier in all received random access response messages does not correspond to the transmitted random access preamble, then it means that the receiving of the random access response message is failed. The failure of receiving of the random access response message means the failure of the random access attempt, and the terminal determines the time for a next random access attempt according to the backoff value if the random access preamble transmission maximum (PREAMBLE_TRANS_MAX) is not reached.
(3) Random Access Response-Scheduled Transmission
For the contention-based random access procedure, after the terminal successfully receives the random access response message in step 2, the time interval from the moment that the subframe indicated by the uplink grant of the message is received to the moment that the terminal transmits the scheduled transmission message in step 3 on the uplink-shared channel is greater than or equal to 6 ms.
(4) Scheduled Transmission-Contention Resolution
The terminal activates a contention resolution timer after transmitting the scheduled transmission message in step 3. During the operation of the timer, the terminal monitors the PDCCH to receive the contention resolution message in step 4. If the terminal successfully receives a corresponding C-RNTI or a temporary C-RNTI and other relevant messages are consistent, then the timer is stopped and the contention resolution is regarded as successful, i.e. the random access procedure is successful, otherwise, the contention resolution is regarded as failed; if the timer is overtime, the contention resolution is also regarded as failed. The failure of the contention resolution means the failure of the random access attempt. The terminal determines the time for a next random access attempt according to the backoff value before the random access preamble transmission maximum (PREAMBLE_TRANS_MAX) is reached. The typical configuration of the length of the contention resolution timer is (8, 16, 24, 32, 40, 48, 56 and 64), taking the ms as a unit.
(5) Random Access Attempt
A random access attempt is from the moment that the terminal transmits the random access preamble in step 1 to the moment that the random access attempt is successful or failed. If the HARQ is needed for many times in step 3, the procedure of the HARQ is regarded as the random access attempt. A successful random access attempt also means a successful random access procedure.
A random access procedure is from the moment that the random access preamble in step 1 is transmitted from the terminal for the first time to the moment that the random access procedure is successful, or from the moment that the random access preamble in step 1 is transmitted from the terminal for the first time to the moment that the random access preamble transmission maximum (PREAMBLE_TRANS_MAX) is reached, wherein it may contain many random access attempts and backoff values between random access attempts. After a random access attempt has failed, the terminal determines the time for next transmission of the random access preamble according to the backoff value to make a new random access attempt. After a random access procedure has succeeded, or the random access preamble transmission maximum is reached, the terminal determines the time for transmission of the random access preamble according to a new triggering event to make a new random access procedure.
According to the above analysis, the time length of a random access attempt is related to the length configuration of a random access response window, the length configuration of a contention resolution timer, the configuration of the HARQ transmission maximum in step 3, and the time of actual receiving of a random access response and a contention resolution message.
Besides the factors as above, the time occupied by a random access procedure is also related to the configuration of the backoff value and the configuration of the random access preamble transmission maximum; a high layer, i.e. an RRC layer, may also direct an MAC layer and a physical layer to terminate the random access procedure. If the length of the random access response window is configured as 10 ms and the length of the contention resolution timer is configured as 32 ms, for the non-contention-based random access procedure in the FDD mode, the maximum time length of a random access attempt is about 2 ms+10 ms=12 ms; and for the contention-based random access procedure in the FDD mode, the time length of a random access attempt is about 2 ms+10 ms+6 ms+32 ms=50 ms. In most cases, a random access attempt may succeed to complete a random access procedure, if the system is not overburdened. In a few cases, a random access procedure may succeed after many random access attempts, or still fail at last, wherein the maximum time is related to the real situation.
In the LTE system, when the terminal conducts inter-frequency or inter-RAT measurement (e.g. when the quality of a service cell is lower than a configured threshold), it needs a measurement gap to perform gap-assisted measurement. During the measurement gap, the terminal cannot monitor the PDCCH and the DL-SCH, nor could it implement transmission on the UL-SCH. The base station configures, activates and/or deactivates measurement gap parameters for the terminal through radio resource control (RRC) signaling. The length of the measurement gap is 6 ms or 8 ms, and the cycle is 40 ms or 120 ms (wherein after a cycle of 120 ms, the cycle may be changed to 80 ms, 128 ms or 160 ms later).
At present, in the 3rd generation partnership project (3GPP) RRC protocol 36.331 v8.2.0, a measurement gap configuration (MeasGapConfig) cell is contained in a measurement configuration cell. The MeasGapConfig cell contains a gap activation cell, which further contains two cells: an activate cell and a deactivate cell; the activate cell further contains three cells: a gap pattern (gapPattern) cell, a start system frame number (startSFN) cell and a start subframe number (startSubframeNumber) cell. It can be seen from the above-mentioned cell structure that the configuration and activation of the measurement gap take effect at the same time. After the measurement gap is configured and activated, the base station and the terminal should keep synchronistic in terms of operation of the measurement gap, and the base station should avoid scheduling uplink or downlink transmission of a corresponding terminal (including feedback information) during the measurement gap.
If the terminal is configured with a measurement gap which is activated, a collision may take place between the random access procedure and the measurement gap, i.e. an overlap may occur between the two in terms of time. For example, if the duration of a random access attempt is about 50 ms, the cycle of the measurement gap is 40 ms, the time within which some steps of the random access procedure happens may overlap with the measurement gap, and the overlap may happen in several early or later steps of the random access procedure.
According to the provisions of the existing 3GPP protocol, as a terminal cannot monitor a PDCCH and implement uplink transmission during a measurement gap, the terminal cannot implement relevant steps of a random access procedure, thereby leading to the failure or significant delay of the random access procedure. In many cases, a LTE system sets a strict requirement on the time of the random access process, e.g. in condition of handover or arrival of signaling data. However, there is not yet a solution for the problem of a collision between a random access procedure and a measurement gap.